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Biology - It Grows on You. : Plant Transpiration Activity, Lab, and Questions

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Biology - It Grows on You. : Plant Transpiration Activity, Lab, and Questions

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In class we have been learning about plant evolution, photosynthesis, and now transpiration. For homework, we did an online lab that tested the different amounts of transpirations in 9 different plants. The transpiration levels were tested using a heater, fan, and light as possible factors of influence. All of these environmental factors effected each of the plants by increasing their rate of transpiration.
The picture below is what was used in the lab to help determine the mL of water.

The results of the 9 different plants are posted down below in the table.

After completing the lab, the website offers more information regarding plant transpiration:

" What factors affect the transpiration rate in plants?
In vascular plants, water is absorbed through the roots and carried upward through the stem to the leaves. The force behind this upward movement is called capillary action, a force of attraction between molecules that causes liquids to move up narrow tubes, such as those inside a plant's stem.
Some of the water absorbed by a plant's roots is used for photosynthesis, but much is lost to the environment through a process called transpiration. During photosynthesis, tiny pores on the surface of the leaves, called stomata, open to permit the intake of carbon dioxide and the release of oxygen. Because the stomata must remain open for the exchange of gases, large amounts of water are lost to the environment through evaporation.
Water that evaporates from the leaves is continually replaced with water that is absorbed through the roots. Therefore a plant's rate of transpiration can be measured by observing the amount of water taken up through a plant's roots over a period of time. The transpiration rate can be approximated by measuring the amount of water taken up in a short time through the plant's stem.
In a laboratory, a plant's transpiration rate can be measured using a potometer. A potometer can be assembled from standard laboratory materials including: a ring stand, clamps, a 10mL pipette, a 100mL burette, a T-tube, glass tubing, and rubber tubing.
To measure transpiration rate, a plant sprig is mounted on the potometer and the burette and pipette are filled with water. Over time the plant will transpire and absorb water through its stem. The potometer is constructed in such a way that the plant's water source is the pipette, therefore the amount of water transpired over time can be determined by reading the water level in the pipette after time has passed. The water supply in the pipette can be replenished from the water supply in the burette by releasing the pinch clamp." ( Information Source)

Lastly, we were asked to complete the journal questions that followed up the lab.

1.) Describe the process of transpiration in vascular plants.

Transpiration is the release of water from a plant, as the water evaporates into the atmosphere from the leaves and stems of plants. In transpiration, water travels from the soil into the roots of plants, and up to the underside of plant leaves, where it is released into the air. This evaporation of water from plants into the atmosphere happens due to the small pores in the leaves called stomates. Stomates are small openings underside leaves that are connected to tissues of a vascular plant.

2.) Describe any experimental controls used in the investigation.

The controls for the plants were by testing the normal amounts of transpiration in the plant without the influence of any environmental factors.

3.) What were environmental factors that you tested increased the rate of transpiration? Was the rate of transpiration increased for all plants tested?

All of the factors that were tested (heat, wind, light) influenced the rate of transpiration by increasing it. In the data, all of the plants showed the same increase with these factors.

4.) Did any of the environmental factors (heat, light, or wind) increase the transpiration rate more than the others? Why?
The heat increased the rate of transpiration in the plants the most. This is because the heat acted as a type of "catalyst" to speed of the rate in which the plant transpires.
5.) Which species of plants that you tested had the highest transpiration rates? Why do you think different species of plants transpire at different rates?

Based on the data collected, the rubber plant had the highest rate of transpiration. Different amounts of water found in soil can influence this, as well as the plant's own metabolic rate. 6.) Suppose you coated the leaves of a plant with petroleum jelly. How would the plant's rate of transpiration be affected?

7.) Of what value to a plant is the ability to lose water through transpiration? The jelly would impact the plant's rate of transpiration by slowing it down significantly. This would occur because the jelly would block the plant's stomata on its underside, and now allow the water in the plant to evaporate.
Because water has cohesion between its molecules, the water is "pulled up" through the xylem. When the water evaporates, this creates the transpiration flow through the xylem and carries any dissolved nutrients with the water itself. For plants, h aving the ability to draw water and nutrients upwards to branching leaves allows plants to spread its leaves to receive more necessary sunlight.

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Plant Transpiration Activity, Lab, and Questions

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In class we have been learning about plant evolution, photosynthesis, and now transpiration. For homework, we did an online lab that tested the different amounts of transpirations in 9 different plants. The transpiration levels were tested using a heater, fan, and light as possible factors of influence.

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Measuring rate of water uptake by a plant shoot using a potometer | Nuffield Foundation

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Measuring rate of water uptake by a plant shoot using a potometer | Nuffield Foundation

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Class practical

In this activity the rate of water uptake, due to transpiration, by a shoot from a woody plant, is measured by timing how long a bubble takes to move a set distance, in the capillary tube of an h-shaped potometer.

Lesson organisation

Decisions will depend on the availability of suitable potometers. This could be a demonstration or an experiment for groups of up to three individuals - observer, timekeeper and scribe.

Apparatus and Chemicals

Lamp

Light meter

Fan

Anemometer

Vaseline or nail varnish

Graph paper

Plant material ( Note 1)

Potometer - designs vary

Scalpel or scissors, 1

Marker pen, 1

Stop clock, 1

Beaker

Water

Plastic ruler

Paper towels

Food colouring, optional ( Note 2)

Health & Safety and Technical notes

Some people find sap from plants irritating to the skin.

Take care when cutting the plant shoot.

Take care when assembling and handling the glass potometer: it is easy to break the long glass tubes and cut or stab yourself with the broken ends. Be prepared with first aid for cuts from broken glass, and brief students how to deal with breakages.

1 Use shoots from a shrub or tree with leaves that have thin waxy cuticles, such as. beech, lilac, Spirea. Leaves with thicker waxy cuticles (very glossy leaves) do not work as well.

2 You must cut the shoots under water and you must assemble the potometer under water. This requires a large sink. If air gets into the xylem vessels of the plant, it can form air locks which will prevent the plant taking up water and so prevent steady transpiration.

3 The potometers should be left for the leaves to dry. Alternatively dry the leaves gently with a paper towel. The potometer will not work properly until any excess water on the leaves has evaporated or been removed.

4 Assemble the potometers before the lesson, since there is a real art in setting them up. This will give the leaves time to dry, and give technicians a chance to check they are working before the students begin to take measurements.

5 Adding food colouring to the water makes it easier to see the air bubble in the capillary tube.

Ethical issues

There are no ethical issues with this procedure.

Procedure

SAFETY: Take care when handling this apparatus as glass tubing is quite fragile.
Make sure students know what to do if anything gets broken.
Have first aid kit ready for cuts.

Preparation

a Set up the apparatus as in the diagram.

b Leave undisturbed so that the shoot equilibrates to the conditions.

c Starting about 2 cm from the free end of the capillary tubing, mark the tubing at 1 cm intervals using the ruler and marker pen. Make as many marks as possible (at least 6).

Investigation

d Introduce a bubble into the capillary tubing by lifting the whole potometer upwards. To do this, loosen the screw on the boss and slide the boss up the clamp stand so that the capillary tube comes out of the water in the beaker. Retighten the screw on the boss.

e Gently blot the end of the capillary tube with a piece of paper towel; an air bubble should appear in the capillary tube.

f Loosen the screw on the boss and lower the potometer, so that the capillary tube just goes back into the water in the beaker. Retighten the screw on the boss.

g There are two ways of taking measurements:

Start the stop clock when the bubble of air touches the first marked line. Stop the clock when the bubble has travelled a fixed distance (2 or 3 cm) and touches the appropriate marked line.

OR:

Start the stop clock when the bubble of air touches the first marked line. Allow the bubble to travel upwards for a fixed period of time. Depending on the rate of movement, this could be 10 or 30 seconds or longer. Mark how far the bubble has moved with a different coloured pen. Measure the distance between the first mark and this second coloured mark.

h Calculate the rate of movement of the air bubble (and hence rate of transpiration) using the formula given here. SI units are cm/s.

Distance moved by the air bubble/Time taken for the air bubble to move that distance

i Change a factor that might affect how quickly the plant loses water by transpiration - see teaching notes.

Teaching notes

Note that many plants close their stomata when the light intensity is insufficient for photosynthesis. Stems harvested in the autumn or winter may not transpire very much. You can use a high-intensity light to promote photosynthesis and get transpiration going again. Fluorescent strip lights or halogen lamps are better than ordinary bench lamps.

You could give students guidance on investigating a particular factor affecting the rate of transpiration, or they can choose from a list and develop their own ideas. Use five or six values for each variable tested.

Light intensity. Place the plant shoot and potometer at different positions round the laboratory to alter the light intensity. Or you could use a lamp set at different distances from the plant. Check the intensity of light reaching the plant by placing a light meter by the leaves of the shoot.

Wind speed. Using a fan to create different wind speeds, either by setting the fan to different speeds or by placing the plant shoot and potometer at different distances from the fan. Check the wind speed at the plant with an anemometer placed just in front of the plant at leaf height.

Temperature. It may be possible to find places which are at different temperatures; but it may not be easy to ensure that all other variables are controlled.

Surface area. There are at least two ways of estimating the effect of surface area. An added extra dimension would be to try to establish if the upper or lower surface is more important in transpiration. Stomata are usually more abundant on the lower surface of leaves.

Estimating effect of surface area

Method 1: Measure the water uptake by the shoot. Remove a number of leaves from the shoot and measure the rate of water uptake again. Keep removing leaves until all the leaves are off the plant shoot. Label the leaves as you remove them, then estimate their area by placing on squared paper and tracing their outlines. Add together the areas of the leaves to find the total area and multiply by two to get the total surface area, as each leaf has an upper and a lower side. Add the leaf areas together in the reverse order that they were removed to get figures for the change in the surface area of leaves attached to the plant shoot.

Method 2: Measure the water uptake by the shoot. Use vaseline - or nail varnish, but this could damage the leaves - to cover one or both sides of a leaf. Test again. Make a note of the order of vaselining/ varnishing the leaves. Calculate the surface area of the leaves as above. This could get messy!

Health and safety checked, September 2008

Downloads

Download the student sheet Measuring rate of water uptake by a plant shoot using a potometer (57 KB) with questions and answers.

Related experiments

Estimating rate of transpiration from a plant cutting

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Measuring rate of water uptake by a plant shoot using a potometer

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Class practical In this activity the rate of water uptake, due to transpiration, by a shoot from a woody plant, is measured by timing how long a bubble takes to move a set distance, in the capillary tube of an h-shaped potometer. Lesson organisation Decisions will depend on the availability of suitable potometers.

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